Approximately four years ago, a group at the Lawrence Livermore National Laboratory (LLNL), led by William F. Krupke, demonstrated [1-4] an optically-pumped atomic Cs laser operating on the resonance line at 894.6 nm (in vacuum, 894.3 nm in air). This transition had lased previously but Krupke et al.'s results were novel in that they optically pumped the 6p 2P3/2 state by exciting the 6s 2S1/2→6p 2P3/2 (D2) transition at λ˜852.3 nm (in vacuum, 852.1 nm in air), as shown in FIG. 1, and relaxed the 2P3/2 state to the lower-lying 6p 2P1/2 level in order to obtain lasing on the 2P1/2→2S1/2 (ground) transition at 894.6 nm (D1 transition, FIG. 1). This scheme for the Cs laser is also illustrated in FIG. 2.
Recently, this pumping scheme has produced more than 10 W of output power at 894 nm and the level of interest in the laser community is rising rapidly because it appears that this laser may offer a route to extremely high power levels. The primary reason for the interest is that it allows one to use high power semiconductor laser diodes as the pump source to drive a gas laser.
Gas lasers are ideal for high power lasers because the index of refraction of the gain medium is small and, hence, obtaining high quality (near diffraction-limited) output beams is generally straightforward. Furthermore, the aperture (transverse dimension) of gas lasers can be scaled readily, an essential feature if high power operation is to be obtained.
All of this is quite attractive but Krupke's pumping schemes have significant drawbacks. Since the atomic transition that is being pumped is spectrally very narrow (≈10 GHz, or equivalently≈0.02 nm), only a small portion of the semiconductor laser power will be absorbed by the alkali vapor because common semiconductor lasers typically emit with spectral widths of >1000 GHz (roughly 2 nm). To surmount this difficulty, Krupke proposed adding He gas (or other gases) to broaden the linewidth of the transition [1-4]. Unfortunately, to do this with He (which has a pressure broadening coefficient of approximately 20 GHz/atm at a wavelength of 800 nm), one must add up to 25-50 atmospheres (19,000-38,000 Torr) of gas if the pump transition linewidth is to match the spectral breadth of the semiconductor laser. In addition, Krupke was forced to use axial pumping in which the small wing absorption is multiplied by the longer axial length to enhance the pump utilization. However, such a pumping scheme has an inherent non-uniform pumping rate and, consequently, a spatially non-uniform population inversion that adversely affects beam quality and the ability to effectively scale in power. The alternative is to narrow the linewidth of the pump laser. This dramatically increases the cost and, more importantly, reduces the electrical-to-optical conversion efficiency because narrow linewidth diode lasers are inherently less electrically efficient than their broader linewidth counterparts. Furthermore, even if one overcomes the reduction in conversion efficiency of the semiconductor pump laser, it is generally necessary to stabilize the wavelength of the pump laser against drift. That is, because the pumping transition and semiconductor laser linewidth are both extremely narrow, “locking” the laser onto the absorption line is usually required. This restriction involves both optical and electronic hardware and results in more electrical efficiency losses.